CS4514 Assignment 2 Help Session – Data Link Layer Client and Server Processes

1. CS4514 Assignment 2 Help Session– Data Link Layer Client and Server Processes Speaker: Hao Shang Date: Nov. 11th, 2004

2. Goal • Implement data link layer based on PAR protocol (Positive Acknowledgement with Retransmission) • PAR: a working stop-and-wait protocol over un unreliable channel. Ref to Figure3-12 in textbook. • Error detection: 1 byte xor • Framing: 80bytes max payload, flag bytes [0x7E] with byte stuffing • Physical layer is emulated by a TCP connection plus an error module.

3. Framework Client Server Network Layer Network Layer Data Link Layer Data Link Layer Physical Layer Physical Layer

4. Network Layer Client Server testdata.raw server.out read (pkt) write (pkt) Network Layer Network Layer nwl_recv(pkt) pkt pkt

5. Data Link Layer Client Server Network Layer Network Layer pkt dll_send(pkt) Data Link Layer Data Link Layer dll_recv(frm) frm ack

6. Physical Layer Client Server Datalink Layer Datalink Layer ack frm phl_send(frm) phl_send(ack) Physical Layer Physical Layer phl_recv(ack) phl_recv() TCP Connection

7. Testdata File • Pkt_num the number of packets • Packet_i_len the byte number of the i-th packet • Packet_i the i-th packet in raw byte form 2 {one byte} 38 {one byte} CS4514, computer network course, FL320 {38 bytes} 31 {one byte} Worcester Polytechnic Institute {31 bytes}

8. Example: Read testdata_simple.raw • ccc:/cs/cs4514/pub/proj2test/getData.c main(int argc, char **argv) { int fp, i; unsigned char packets[221]; unsigned char byteNum; unsigned char p; if ((fp = open(argv[1], O_RDONLY)) < 0) { fprintf(stderr, "Open testData error!“); printf("Usage: %s filename\n", argv[0]); exit(-1); } read(fp, &p, 1); printf("The total number of packets is: %d\n\n", p);

9. Example: Read testdata_simple.raw (continued) for (i = 0; i < p; i++) { read(fp, &byteNum, 1); printf("The length of %dth packet : %d\n", i+1,byteNum); read(fp, packets, byteNum); //the following two lines are applicable for printable packets only packets[byteNum] = '\0'; printf("The content of %dth packet : %s\n\n", i+1,packets); } close(fp); } • Raw file: ccc:/cs/cs4514/pub/proj2test/testData_simple.raw • Another example: ccc:/cs/cs4514/pub/proj2test/getData_v1.c • NOTE: Packets may not be printable characters

10. Client: dll_send(pkt, …) 1. Split a packet into payloads phl_send(frm, …): Force bit error every 6-th Frame 2. For each payload 2.1 Create Frame (frm) 2.2 phl_send(frm, …) 2.3.1 Timeout Handler: phl_send(frm, …) restart the timer 2.3 Start a Timer 2.4 phl_recv(ack, …) ack ok? Stop the timer no client.log yes

11. Create Frame Datafield 1. Compute Seq Number and End-Of-Packet (EOP) byte Seq EOP Datafield 2. Error-Detection (ED) byte (XOR on Seq + EOP + Data) Seq EOP Datafield ED 3. Byte Stuffing on Seq + EOP + Data + ED Bytes after stuffing 4. Add Frame-Flag (FF) 0x7E at both ends FF Bytes after stuffing FF EOP: End of Packet ED: Error Detection FF: frame flag

12. Server: dll_recv(frm, …) 1. Un-stuff frm phl_send(frm, …): Force bit error every 5-th Frame 2. Compute ED byte ED ok? Return no yes server.log Dup? Drop frm yes no 3. Create ACK Frame (ack) 4. phl_send(ack, …) no EOP? 5. Reassemble the packet yes 6. nwl_recv(pkt, …)

13. Create ACK Frame 1. Compute Seq Number Seq 2. Error-Detection (ED) byte (ED = Seq) Seq ED 3. Byte Stuffing on Seq+ED Bytes after stuffing 4. Add Frame-Flag (FF) 0x7E at both ends FF Bytes after stuffing FF EOP: End of Packet ED: Error Detection FF: Frame Flag

14. Timers • The client uses a timer to detect a frame loss. • The client sets a timer when it transmits a frame. • When the timer expires, the client retransmits the frame. • Two kinds of timer • Select : block • Signal and Timer: non-block

15. Select: Monitor Given FDs (SDs) # include <sys/select.h> # include <sys/time.h> int select (int maxfdp1, fd_set *readset, fd_set *writeset, fd_set *exceptset, const struct timeval *timeout); struct timeval { long tv_sec; /* seconds */ long tv_usec;/* microseconds */ }

16. Example: Select fd_set bvfdRead; int readyNo; struct timeval timeout; int sockfd; while (1) { timeout.tv_sec = 0; timeout.tv_usec = 500; FD_ZERO(&bvfdRead); FD_SET(sockfd, &bvfdRead); readyNo = select(sockfd+1, &bvfdRead, 0, 0, &timeout); if(readyNo < 0) error_handler(); else if(readyNo == 0) timeout_handler(); else { FD_ZERO(&bvfdRead); receive_handler(); } }

17. Signal and Timer: Soft Interrupt • Head files #include <signal.h> #include <time.h> • Register a function to TIMEOUT signal signal (SIGALRM, timeout); • Create a timer and begin to run timer_create(); timer_settime(); • Compile with option “-lrt” (link runtime library)

18. Example: Signal and Timer timer_t timer_id; void timeout(int signal_number){ printf("\n SIGNUM: %d\n", signal_number); exit(0); } void start_timer(){ struct itimerspec time_val; signal (SIGALRM, timeout); timer_create( CLOCK_REALTIME, NULL, &timer_id); /* set timeout to 1 second */ time_val.it_value.tv_sec = 1; time_val.it_value.tv_nsec = 0; /* set both fields of it_value 0 to disarm the alarm */ time_val.it_interval.tv_sec = 0; time_val.it_interval.tv_nsec = 0; timer_settime(timer_id, 0, &time_val, NULL); } main(){ start_timer(); while(1); }

19. Questions Thank You !